If the voltage controlled oscillator applies to communication equipment, the oscillator needs to have a function of adjusting a free-running frequency.
Two typical high-frequency voltage controlled oscillators are shown in FIGS. 2 and 3, respectively. The function of the oscillator will be described as referring to the drawings. Those voltage controlled oscillators both are a clap type. As shown, 1 is an input terminal at which a frequency-controlling voltage is applied. 2 is a varactor, 3 is a trimming stub for adjusting a free-running frequency, 4 is a microstrip line resonator, 5 is a supply-voltage terminal, 6 is an output terminal, and 7 is a trimmer capacitor for adjusting a free-running frequency.
At first, a supply voltage is applied at the terminal 5 and a voltage for controlling a frequency is applied at the terminal 1. The application of these voltages provides the varactor 2 with a constant capacitance C.sub.4. At this time, the voltage controlled oscillator has an oscillating frequency represented by: ##EQU1## Hence, by adjusting the voltage applied to the varactor C.sub.4, it is possible to vary the oscillating frequency. In the operation, the microstrip line resonator 4 serves as inductance L.sub.1.
The frequency-controlling voltage applied at the terminal 1 has a predetermined range so that the free-running frequency is required to be adjusted according to the range. For adjusting the frequency, in the voltage controlled oscillator shown in FIG. 2, the oscillating frequency may be varied by adjusting C.sub.5 with the trimmer capacitor 7, because the change of the trimmer capacitor 7 equally leads to change of C.sub.4. In the voltage controlled oscillator shown in FIG. 3, on the other hand, the oscillating frequency may be varied by adjusting the length of the microstrip line with the trimming stub 3.
However, the former voltage controlled oscillator needs the trimmer capacitor 7 having a capacity for the wide range of the oscillating frequency. The critical performance of the trimmer capacitor results in limiting the realized oscillating frequency to about 1.5 GHz. Further, the latter voltage controlled oscillator has difficulty in minutely varying the frequency because the length of the microstrip line resonator 4 is adjusted by the trimming stub 3 for adjusting the oscillating frequency. The oscillator has additional difficulty in selecting one of the increase and decrease of the frequency.
To solve the difficulties, it is an object of the present invention to provide a high-frequency voltage controlled oscillator which has a trimming stub for adjusting a free-running frequency and the microstrip line resonator located in parallel to the trimming stub.